I. Field of the Invention
The present invention relates to a power factor controller for use with an alternating current power distribution system.
II. Description of the Prior Art
In industrial applications, the electrical load requires not only kilowatts (KW) or working current but also kilovars (KVAR) which are drawn by motors and other inductive electrical equipment which require magnetizing current. The kilovar requirement increases proportionately with the inductive load and also proportionately increases the phase shift between the voltage and current components of the electrical power in the power distribution system.
The power factor for an alternating current power distribution system is equal to the cosine of the phase shift between the voltage and current signal components of the alternating electric power. The power factor is inversely proportional to the phase shift between the voltage in current and thus decreases as the phase shift increases.
A low power factor caused by high inductive electrical loads is disadvantageous in that it results in poor electrical efficiency. Consequently, a low power factor results in power losses in the individual motor feeders and in stepdown transformers. A low power factor also increases the resistive heat loss in transformers and other electrical distribution equipment and increases the difficulty of obtaining proper voltage stabilization. Perhaps more importantly, however, many electric utility companies impose penalties on industrial users when the power factor for the user falls below a prescribed amount. Consequently, a lower power factor ultimately results in increased electric utility bills.
In order to increase the power factor, it has been a previously known practice for industrial users to couple power capacitors in shunt with the inductive load since the capacitor reactance vector opposes the inductive reactance vector. Moreover, there have been several previously known power factor controllers which continually determine the power factor or phase shift between the voltage in current and progressively electrically connect capacitors from a bank of capacitors to the inductive load in order to increases the power factor. If the inductive load subsequently decreases, these previously known power factor controllers disconnect the capacitors from the inductive load in the reverse order, i.e., the last capacitor to be electrically connected to the load is first to be electrically disconnected and so on throughout the capacitor bank.
These previously known power factor controllers, however, have suffered from a number of disadvantages. One disadvantage of the previously known power factor controllers is that the on time for the individual capacitors, i.e., the duration of time that the capacitor is electrically connected with the inductive load, varies greatly from capacitor to capacitor. This occurs because the last capacitor to be electrically connected to the inductive load is the first to be electrically disconnected from the inductive load if the inductive load decreases and vice versa. Consequently, in a bank of N capacitors, the first capacitor in the bank in all likelihood will be electrically connected to the inductive load substantially constantly while the last capacitor in the capacitor bank is only occasionally, if ever, connected to the inductive load. Such uneven on time for the power capacitors disadvantageously results in an early failure for the capacitors at the beginning of the capacitor bank and also inefficient use of the capacitors at or near the end of the capacitor bank.
A still further disadvantage of these previously known power factor controllers is that they utilize a mechanical switching arrangement in order to electrically connect one disconnect the capacitors from the inductive load. Such mechanical switches, however, not only require periodic maintenance but are also prone to failure and thus necessitate replacement. Moreover, while the repair or replacement on the mechanical switches is performed, the power factor controller must be electrically disconnected from the inductive load for the obvious safety reasons.
A still further disadvantage of the previously known power factor controllers is that such power factor controllers attempt to obtain a unity power factor by selectively connecting and disconnecting capacitors to the inductive load. There is little reason, however, to maintain a power factor greater than the power factor below which the electrical utility charges a penalty. Typically, no penalty is assessed against the industrial user as long as the power factor remains above typically 0.8-0.9 and an attempt to maintain a unity power factor results in unnecessary use of the capacitors. Such capacitors, moreover, have a limited life and their unnecessary use results in their premature failure.